With the increasingly high integration and greater sizes of semiconductor elements in recent years, there have arisen demands for thinner and lighter-weight package boards. This has in turn led to demand for formation of insulating layers by materials exhibiting more excellent electrical characteristics, heat resistance and mechanical properties, in package boards with semiconductor element surface protective layers, interlayer insulating films or redistribution layers (hereunder also referred to as “semiconductor devices”). Polyimide resins are materials that can satisfy such demands, and research has been conducted on the use of photosensitive polyimides, for example, that are polyimide resins imparted with photosensitive properties. Using a photosensitive polyimide has the advantage of simplifying the pattern formation steps and allowing complex production steps to be shortened (see Patent documents 1 and 2, for example).
Cured films of polyimide resins are generally formed by producing a thin-film by a method such as spin coating of a solution (or “varnish”) of a polyimide precursor (polyamide acid) obtained by reacting a tetracarboxylic acid dianhydride with a diamine, and performing thermal dehydrating cyclization (see Non-patent document 1, for example). The polyimide resin is cured through the process of dehydrating cyclization. However, when a polyimide resin is obtained using a polyimide precursor, volume shrinkage occurs due to dehydration (imidation) during curing, and this can cause problems such as loss of film thickness and reduced dimensional precision. Furthermore, lower-temperature film-forming processes are desired in recent years, and there is a demand for polyimide resins that are capable of dehydrating cyclization at low temperature, while exhibiting physical properties after dehydrating cyclization that are comparable to those of films obtained by dehydrating cyclization at high temperature. However, curing of polyimide precursors at low temperature results in incomplete imidation and therefore reduced physical properties, including brittleness of the produced cured film.
Another subject of study is photosensitive resins that employ other polymers with high heat resistance, and that do not require the dehydrating cyclization as polyimide precursors (see Non-patent document 2 and Patent documents 3-7, for example).